<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:math="http://www.w3.org/1998/Math/MathML">
      
<body onload="boom();">

<script>
function boom()
{
  document.getElementById("merror").appendChild(document.getElementById("s"));
}
</script>

<math:merror id="merror"></math:merror>

<span id="s" />

</body>
</html>

